Jesus proclaimed how believers in Jesus also believe in God and how people who reject Jesus have their unbelief against them. These proclamations are important, and Jesus would explain why such proclamations are important.
The short answer -- Jesus's words are from God the Father. Jesus says he did not speak "on [His] own authority" but rather followed the Father's command on speaking (12:49). In fact, whatever the Father tells Jesus, Jesus speaks (12:50). Jesus also told people that the Father's command is everlasting life (12:50). So everything Jesus said about believing and everlasting life is originally from the Father!